1. Field of the Invention
This invention relates to an apparatus for converting light signal into digital electrical signals.
2. Description of the Prior Art
In an apparatus such as spectrophotometer for analyzing a specimen by use of light, the amount of light is converted into an electrical signal by such means as a phototube or photoconductive cell, and the electrical signal is processed to determine such properties as transmittance, absorbance and reflectance of the specimen. With the progress of the digital techniques, it is become possible now to process the electrical signal digitally as disclosed in U.S. Pat. No. 3,880,524 issued Apr. 29, 1975 to Frederic H. Dill et al and copending U.S. patent application Ser. Nos. 836,955 filed Sept. 27, 1977 and 843,790 filed Oct. 20, 1977 which were also assigned to the assignee of the present invention. In order to process the electrical signal digitally, the analog electrical signal produced from photo-electric converter means is converted into a digital signal by an analog-to-digital converter (hereinafter referred to as the A-D converter). For processing the digital electrical signal, the amount of light is required to be converted into an exact amount of corresponding digital electrical signal. Further, in fluorescence spectrometry, Raman spectrometry or measurement of absorbance of a specimen covering a wide range from low concentration to high concentration, not only a wide dynamic range is necessary but also even a very weak ray of lighr must be accurately converted into a digital electrical signal. Such an accurate conversion, however, is impossible by the conventional apparatuses for several reasons.
Firstly, the linear relation between input analog signal and output digital signal of the A-D converter is poor for very weak input signals. An A-D converter takes the form of a voltage-frequency transducer (hereinafter referred to as the V-F transducer) or double integrator. The satisfactory linearity of the V-F transducer is limited to about 1% of input full scale, depending on the pulse frequency converted. The linearity of the double integrator used as the A-D converter, on the other hand, depends on the accuracy of a level comparator and is satisfactory only in the range from about 0.5% to 5% of input full scale. In other words, the satisfactory lineariy of an A-D converter 10 V in full scale is exhibited in the range only from 100 mV to 10 V, so that a very weak signal less than 100 mV cannot be accurately converted into a digital electrical signal.
Secondly, an electrical signal produced from the photo-electric converter means contains various noise components including those generated by the photo-electric converter means itself and peripheral circuits. The noise components are electrically positive or negative. In the A-D converter processing only positive electrical signals, however, those weak electrical signals in the vicinity of zero level containing a large amount of noise are ignored, thus making it impossible to produce a digital amount accurately proportional to the amount of light. The noise signal level depends on various conditions, but generally ranges from about 1 mV to 1 V.
Thirdly, an undesirable DC signal component is contained in the electrical signal proportional to the amount of light, which is produced from the photo-electric converter means. This DC signal component varies with temperature. An example of the undesirable DC signal component is the dark current in a phototube or photo-multiplier, or dark resistance in the case of a photoconductive cell such as PbS cell. An offset voltage or temperature drift in the amplifier is another example. The dark resistance changes by 20 to 30% with a temperature change of about 10.degree. C. This DC signal component makes it impossible to produce a digital electrical signal in exact proportion to the amount of light.